South Korea is doing its utmost to hold multilateral talks on North Korea's nuclear programme within two months, according to a top official.
The South Korean President's foreign policy chief, Ban Ki-moon, told YTN television that the venue could be determined after all involved agree on such talks.
North Korea has been insisting on holding direct talks with the United States, but Washington says any talks should involve South Korea, Japan, Russia, and China.
The North is thought to have stored 8,000 spent fuel rods
But Mr Ban said the North has shown a gradual change in its stance towards holding multilateral talks.
On Wednesday, Seoul's intelligence agency said that North Korea had reprocessed "a small number" of its spent nuclear fuel rods, South Korea's intelligence agency said on Wednesday.
North Korea has claimed before to have begun reprocessing its 8,000 spent rods - a move which could allow it to extract enough weapons-grade plutonium to develop a handful of nuclear weapons within months of starting the reprocessing operation.
But international intelligence agencies have been unsure whether the North was bluffing, as part of its continuing high-risk stand-off over its nuclear ambitions.
The United States appeared unconvinced by the latest developments, with a US official telling Reuters news agency: "Our assessment of where they are on reprocessing is not 100% clear."
National Intelligence Agency director Ko Young-koo told parliament that his staff had noticed "high-explosive tests being conducted in Yongduk district", which is 40 km (25 miles)
northwest of Yongbyon.
Conventional high explosives can be used as the trigger for nuclear devices.
NORTH KOREA'S NUCLEAR PROGRAMME
Yongbyon: Five megawatt experimental nuclear power reactor and a partially completed plutonium extraction facility. Activities at site frozen under 1994 Agreed Framework
Taechon: 200-MWt nuclear power reactor - construction halted under Agreed Framework
Pyongyang: Laboratory-scale "hot cells" that may have been used to extract small quantities of plutonium
Kumho: Two 1,000-MWt light water reactors being built under Agreed Framework
South Korea's Defence Ministry has already claimed that the North conducted numerous nuclear-related tests of explosive material in the 1980s and 1990s.
The 8,000 spent fuel rods are part of a plutonium-based nuclear weapons programme that was halted under a 1994 nuclear agreement between North Korea and the US and its allies.
The agreement crumbled last year after US officials said Pyongyang had admitted to having a secret atomic programme.